1. Field of the Invention
This invention generally relates to certain aspects of high speed computer parallel databus structures. More particularly, the present invention relates to a novel apparatus and method for improving the transmission of data between a computer and peripheral devices using parallel databus structures.
2. Description of Related Art
Typically, computers will have at least one parallel data bus infrastructure capable of accommodating the digital signals communicated between a host computers"" processor and a peripheral hardware, such as a memory storage device. For example, as illustrated in FIG. 1A, host device 50 transmits digital data to a transmit parallel bus interface 60, which converts the digital data to a form suitable for transmission and transfers the data on the parallel bus 70. At the other end, the digital data is received by a receive parallel bus interface 80, which converts the digital data and transfers it to the memory storage device 90.
Such communications between the host device 50 and the memory storage device 90 may comply with one (or several) of the well-known Small Computer Standard Interfaces (SCSI) protocol. SCSI is a set of evolving electronic interface standards that allow computers to communicate with peripheral hardware in a parallel fashion. In doing so, SCSI interfaces are faster and more flexible than previous interfaces. For example, the Ultra-3 SCSI standard specifies a bus that can transfer data up to 160 Mbytes/sec and can interconnect up to 16 devices. TABLE 1 summarizes various attributes of the currently-adopted SCSI standards, below:
SCSI interfaces are not, however, immune to signal distortion effects that are inherent in most digital transmission apparatuses. At fast digital data transmission speeds or long cable lengths, SCSI interfaces are susceptible to inter-symbol interference (ISI), which distorts a digital signal by temporally spreading and consequently overlapping the individual digital symbols. As digital data transfer speed increases, the ISI effects in communication via a long cable become substantial to the degree where simple voltage level detection becomes insufficient in distinguishing between logic level changes and, thus, the receiver may not be reliable enough to extract data from the incoming signal.
A major contribution to these ISI effects is due to the resistance (R) and capacitance (C) of the cable, which affects both the phase and amplitude of the digital signals conveyed by the cable. In fact, in addition to the resistance of the cable depending on the wire gauge, coating material, and the stranding configuration, the overall attenuation of the cable generally increases exponentially with frequency. Moreover, at longer cable lengths, the digital signal amplitude may vary in accordance with the data pattern. Thus, when evolving SCSI standards propose increasing the transfer rate (e.g., from 160 Mbytes/sec to 320 Mbytes/sec), the frequency components carrying the digital information are doubled, and the R-C effects become even more impairing. Under certain conditions, the isolated digital pulse amplitude can be too low to be reliably detected by a simple comparator, as currently used in SCSI environments
To mitigate these ISI effects, one conventional approach equalizes the digital pulse signal by employing a second order zero (s2) in the frequency domain to narrow the pulse into a pre-defined target shape. Such an approach is effective in reducing the amplitude distortions associated with ISI, but is relatively ineffectual in correcting phase distortion.
Another conventional approach incorporates a matched filter having a transfer function that is the inverse of the transmission media transfer function. In this manner, amplitude and phase distortions induced by the cable are neutralized. Although effective, the application of this approach is limited by the fact that SCSI interfaces utilize multiple signal lines, each of which may have a different cable impedance due to variances in wire gauge, coating materials, stranding arrangement, etc. Moreover, SCSI interfaces must accommodate a host of different apparatus configurations. These factors make it extremely difficult, if not economically unfeasible, to provide a single match filter that adequately covers the various cases.
What is needed is a method and apparatus that can overcome the limitations of conventional approaches by providing an adaptive equalization technique that adaptively amplifies the frequency components that carry the digital data information and attenuates nonessential frequency components to reduce ISI and make data detection reliable. Moreover, the method and apparatus must be simple to implement and economically feasible.
An apparatus and a method utilize an adaptive equalization technique for mitigating inter-symbol interference effects on an oscillating signal from which digital data will be obtained at a receive end of a channel. Such an apparatus may include a filter element for receiving an input signal from a transmitting device and outputting a filtered signal within a predetermined frequency band. The filter element comprises a mechanism for compensating for the transmission channel losses in the signal frequency band, and increasing the rejection of undesired higher frequency signals. The apparatus and method also include an amplitude determining mechanism for determining the amplitude of the filtered signal and a gain control mechanism to adapt the filter characteristic for optimum compensation of the transmission channel losses at each individual receiver location on the parallel bus structure.